Polymers enriched in a .pi.-electron conjugated system have attracted industrial attention due not only to their electroconductive properties, but also due to their unique physical properties, such as their ability to change chemical or physical state upon metal/semiconductor transition, and investigations have been made on these polymers. In particular, although many electroconductive polymers, such as polyacetylene, polythiophene, polypyrrole, and polyparaphenylene are insoluble and infusible due to their rigid main skeletons (see Skotheim, Handbook of Conducting Polymers, Marcel Dekker, 1986), if a substituent, such as an alkyl group, is incorporated into their side chains, these polymers become soluble and easy to process, thus attracting attention in the industry.
Specific known examples of such polymers are a polymer rendered soluble by incorporating a long-chain alkyl group into the side chain of polythiophene (K. Jen et al., Journal of Chemical Society, Chemical Communication, p. 1346, 1986), and a polymer rendered water-soluble by incorporating an alkylsulfonic acid group (A. O. Patil, Journal of American Chemical Society, vol. 109, p. 1858, 1987). Further, a technique for the indirect processing of a rigid polymer by making use of the solubility of its precursor polymer is known, although the resulting electroconductive polymer having a .pi.-electron conjugated system is insoluble and infusible (Murase et al., Polymer Communications, vol. 25, p. 327, 1984).
Among many .pi.-electron conjugated polymers, a bicyclic electroconductive polymer, in particular, a polymer having an isothianaphthenylene structure, has the lowest bandgap as a semiconductor, and is expected to have a high conductivity. Therefore, it is of particular interest because when this polymer is converted into a p-type electroconductor by electrochemical doping, the .pi.-.pi.* absorption moves from the visible region towards the near infrared region, and the polymer becomes a transparent electroconductor (Kobayashi et al., Journal of Chemical physics, vol. 82, p. 5717, 1985). However, polyisothianaphthene, which was first electrochemically synthesized by Kobayashi et al., is an insoluble and infusible polymer and had a processability problem.
Accordingly, various investigations have been carried out in an attempt to make improvements from an industrial standpoint. Reports on some derivatives have already been made. For example, it is assumed from calculation results that when an electron-attracting group or an electron-donating group is introduced into an isothianaphthenylene skeleton, the electron state as a semiconductor is affected (see Bredas et al., Journal of Chemical Physics, vol. 85, p. 4673, 1986). An organic solvent-soluble derivative introduced by a long-chain alkoxy group into the isothianaphthenylene skeleton for improving processability has also been reported (JP-A-2-242816, the term "JP-A" as used herein means an "unexamined published Japanese patent application").
Additional examples include polymers having a halogen as a substituent (JP-A-63-307604), and polymers having an electron-attracting group as a substituent (JP-A-2-252727). However, none of the above-mentioned publications describe any of the characteristics of the polymers of the present invention, nor disclose any specific production process therefor.